1. Field of the Invention
The present invention relates to the technique of ink jet recording, and more particularly to an ink jet recording method in which an ink image is overcoated with a clear ink.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2005-081754 discloses a technique of applying a clear ink onto an ink image, which is formed on a recording medium in ink jet recording, to thereby overcoat the surface of the ink image with the clear ink. The overcoating can increase glossiness of the image and resistance against scratching (hereinafter referred to as “scratch resistance”).
The state of the surface of the ink image before the overcoating differs depending on the type of ink used to form the ink image and the recording density of the ink. For example, as the recording density increases, dots of an ink having a small surface tension generally tend to easily immingle with one another, and the ink image after being fused and fixed forms a relatively smooth surface. On the other hand, dots of an ink having a great surface tension generally tend to form a surface having relatively noticeable irregularities because those dots are fused and fixed while keeping the dot shape. Color development irrelevant to the image may occur when overcoating the ink image that is formed by using the former ink tending to form the smooth surface. An interference color is generated through a mechanism described below.
FIG. 1 is a schematic view illustrating a cross-section of layers including a recording medium when a clear ink is coated on an ink image that is formed by the ink tending to form the smooth surface. An ink image layer 1002 recorded by using the ink is formed on a recording medium 1001, and a clear ink layer 1003 is formed on the ink image layer 1002. The clear ink layer 1003 generally has a thickness d of about 100 nm to 500 nm.
A parallel light 1004 (1004a and 1004b) from the sun or a fluorescent lamp, for example, is separated into a reflected light 1005 that is reflected at the surface of the clear ink layer 1003, and a reflected light 1006 that is reflected at the surface of the ink image layer 1002 after passing through the clear ink layer 1003. Interference occurs between the two separated lights due to a difference in optical path therebetween.
Given, for example, that the incident angle is θ, the wavelength of incident light is λ, and the refractive index of the clear ink layer 1003 is n, the intensity of light having the wavelength λ, which satisfies the relationship expressed by the following formula (1), is increased and an interference color of the relevant light is more strongly visually recognized by an observer:m×λ=n×2d×cos θ+λ/2 (m: natural number)  (1)The wavelength λ satisfying the formula (1) varies depending on a thickness d of the clear ink layer 1003. Therefore, when the thickness of the clear ink layer 1003 is not uniform, the rainbow-colored reflected light may be recognized by the observer in some cases. Such color development irrelevant to an ink image degrades quality of the ink image.
Thus, the ink image having the smooth surface coated with the clear ink is tinted by the interference light having a particular color and is visually recognized as a color tone that has changed from the original color tone of the ink image.
The interference color generated through the above-described mechanism is more conspicuous in a primary color region with a particular ink. In the primary color region, when dots of the same type of ink are applied to the recording medium in closely adjacent relation, those dots tend to easily immingle with one another because of high affinity and to form a smooth ink layer on the surface of the recording medium.